At the present time, there are four main methods used to test the charge of a battery cell:
By far the most common is the method wherein a hydrometer is used to test the electrolyte density, this being the most useful for general purposes since the test is reasonably accurate and can take place under very adverse conditions in the field. However, within a battery cell, stratification of electrolyte takes place, the denser electrolyte remaining near the bottom of the cell and the lighter electrolyte near the top, and because of the construction of most cells, the hydrometer is sensitive only to the upper strata of electrolyte and consequently a battery can, for example, be approaching its fully charged condition, but a hydrometer reading will indicate that it is in a state of greater discharge. This in turn can lead to overcharging of the battery.
Voltage testing in one of two forms can also take place. In one form, the voltage of the cell is sampled, and in this form the test is reasonably accurate but the relationship between voltage and the state of charge is not linear. Another form of voltage check is to check the voltage drop across a sample of electrolyte (this really being a pH check) but this is subject to variation if the electrodes become dirty or contaminated.
Another type of test which has been proposed is one wherein use is made of an electroplate type cell to record the ampere-hours taken out or put into a battery, but there is usually a lag or lead between the actual state of charge and the metal deposit in the cell, and in some instances serious damage has been inflicted upon batteries due to overcharge or over discharge when this method has been used.
A fourth test which has given excellent results in the laboratory has been a test wherein light is passed through a bent tube of glass having a refractive index similar to the refractive index of the electrolyte when in its charged condition, but this test is subject to the disability that the glass surface becomes dirty or contaminated if the test is used under ordinary workshop conditions, and accuracy is lost.
The main object of this invention is to provide a charge sensing means which is sufficiently sensitive to provide an accurate indication of battery charge and sufficiently rugged to provide good results even though conditions are quite adverse.